氮化鎵磊晶層的品質好壞在高電子遷移率電晶體裡扮演著非常重要的角色。因此，我們提出了一個特殊的方法來解決在使用分子束磊晶法成長氮化鎵時，遭遇到的結構以及表面形貌的品質不能兼顧的困境。在這項研究裡，我們先用了一個氮充足的氮化鎵成長條件來成長一層起始變化層；然後，逐步地變化氮/鎵流量比使其慢慢地變成鎵充足的成長條件，隨即接著成長一層磊晶層。在 X 射線繞射的分析中，氮化鎵(002) rocking curve 的半高寬值與純鎵充足樣品比較下，從531.69 arcsecond 改良到59.43 arcsecond。在原子力顯微鏡的分析下，樣品表面的方均根粗糙度與純氮充足樣品比較下，在5 μm × 5 μm 的區域中，從18.28 nm 改進到 1.62 nm 。拉曼光譜實驗指出，有一個微小頃斜的平面存在於梯度變化層裡，並且，這個梯度變化層可以大量消除由於鎵充足氮化鎵與氮化鋁晶格不匹配所造成的應力。

我們使用分子束磊晶法在有機金屬化學氣相沉積法成長的氮化鎵襯墊層成長了一系列不同鋁含量x (x = 0.017~0.355) 的高電子遷移率氮化鋁鎵/氮化鎵異質結構樣品。用 X 射線繞射以及原子力顯微鏡做品質檢測，得到與氮化鎵襯墊層一致的優良的性質。在這一系列樣品中，最高電子遷移率是在8 K 下量測到的 19593 cm2/Vs 伴隨著 3.13 × 1012 cm-2 的載子濃度，同時鋁含量為 x = 0.017。在我們的實驗裡，載子濃度隨著鋁含量的減少而減少。在照光的霍爾量測中，只有少量的電子在藍光發光二極體的照射後增加。這就表示只有少量的深能階缺陷存在於異質界面附近。從變溫的 Shubnikov-de Haas 震盪實驗中，我們可以得到鋁含量 x = 0.207 和 0.136 樣品中的二維電子氣電子有效質量分別為 0.213 mo 和 0.227mo 。

我們使用具焦離子束將高電子遷移率氮化鋁鎵/氮化鎵樣品製作成一系列的奈米線，活性通道的寬度從 900 nm 到 50 nm (900 nm, 500 nm, 300 nm, 200 nm, 100 nm, 80 nm and 50 nm) ，方向為[11 0]方向。在這一系列奈米線中，最大的自旋分裂能量為 2.14 meV 。根據大的自旋分裂能量和類彈道傳輸原則，在我們的例子裡， 200 nm 的奈米線是最適合被用來當作量子環干涉元件的通道的選擇。

The quality of GaN template layer plays a very important role in high electron mobility transistors. We proposed a special method in the growth of molecular beam epitaxy to deal with the dilemma between structure and the morphology of GaN. In our study, we used a nitrogen-rich GaN growth condition to deposit the initial varied layer. After that, we changed the N/Ga ratio stepwise to the growth condition of gallium-rich GaN and grew the epitaxy layer right away. In X-ray diffraction analysis, the full width at half-maximum (FWHM) value of rocking curves of GaN(002) was improved relatively to gallium-rich sample from 531.69 arcsecond to 59.43 arcsecond. In atomic force microscopy (AFM) analysis, the root mean square (rms) roughness of sample surface was improved relatively to nitrogen-rich sample from 18.28 nm to 1.62 nm over 5 μm × 5 μm area. The Raman scattering shows there is a slightly tilted plane in gradient layer and the gradient layer can also slash the strain force which is caused from Ga-rich GaN epitaxy layer and AlN buffer layer.

A series high mobility AlxGa1-xN/GaN heterostructures samples were grown on MOVPE-grown GaN templates substrate by molecular beam epitaxy with different Al concentrations (x = 0.017~0.355). The quality checked by XRD and AFM indicated that the excellent properties agreed with the GaN-template. The highest mobility in this series samples at 8 K is 19593 cm2/Vs with carrier concentration 3.13 × 1012 cm-2 and Al concentration x = 0.017. In our experiments, the carrier density decreases as Al concentration reduces. In the illuminated Hall measurement, there are only few electrons increased following blue LED illumination. It shows that there are only few deep level defects existing near the heterointerface. From temperature-depended Shubnikov-de Haas (SdH) oscillations, the electron effective mass m* in 2DEG are evaluated as 0.213 mo and for x = 0.207 0.227 moand 0.136 respectively.

The high mobility AlxGa1-xN/GaN was fabricated to a series of wires by focused ion beam (FIB) equipment, and the width of the active channel is ranged from 900 nm to 50 nm (900 nm, 500 nm, 300 nm, 200 nm, 100 nm, 80 nm and 50 nm) with the channel orientation in [11 0] direction. The largest spin-splitting energy in the series of wires is 2.14 meV. Due to larger spin-splitting energy and quasi-ballistic transportation, the 200 nm wire is the best candidate to be the channel of the quantum-ring interferometer in our case.

Contents

致謝 iv
Abstract vi
摘要 viii
Chapter 1 Introduction 1

Chapter 2 Improvement of GaN Epilayer by Gradient Layer Method in
Molecular-Beam Epitaxy 5
2.1 Background and Motivation 5
2.2 Growth of Samples 6
2.3 Results and Discussions 8
2.4 Summary 15

Chapter 3 High Electron Mobility AlxGa1-xN/GaN Heterostructures Grown by PAMBE on GaN Templates Prepared by MOVPE 16
3.1 Background and Motivation 16
3.2 Growth and Fabrication of Samples 18
3.3 Results and Discussions 19
3.4 Summary 37

Chapter 4 Spin Splitting in AlxGa1-x/GaN Quasiballistic Quantum Wires 39
4.1 Background and Motivation 39
4.2 Growth and Fabrication of Samples 41
4.3 Results and Discussions 42
4.4 Summary 56

Chapter 5 Conclusions and Future Works 57

References 60

Publication List 64

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